The present invention relates to a display method, particularly, a high speed display method for a nuclear magnetic resonance signal.
Nuclear magnetic resonance (NMR) relates to the fact that when nuclei are placed in a uniform magnetostatic field, they precess around magnetic lines of force at a frequency proportional to an intensity of the field. This frequency, known as the Larmor frequency, is given by .omega..sub.0 =.gamma.H.sub.0, where .gamma. is the gyromagnetic ratio of the nucleus and H.sub.0 is the magnetic field intensity. When the magnetostatic field intensity is varied in a certain direction, nuclei in various positions along the direction precess at different frequencies. When both an oblique magnetic field namely, magnetostatic field with magnetic field gradients and a pulsed RF magnetic field of sufficient magnitude are applied to a sample material, only nuclei having spins which precess at the RF magnetic field pulse frequency are rotated by 90.degree. or 180.degree., and thus they can be isolated from other nuclei.
In British Pat. No. 2,079,946, it is proposed to obtain a two-dimensional image of such a sample using the so-called "spin warp" method. Summarizing the spin warp method, a thin slab of material is assigned in the substance and the latter is disposed in a combination field of a first magnetic field gradient parallel to the slab and a second magnetic field gradient parallel to the slab and orthogonal to the first field gradient. The first gradient is then inverted to detect a free induction signal (FIS). The FIS is produced by spins which are dephased firstly by the first magnetic field gradient and then rephased to form spin echoes. When the spin echos are sampled N.sub.x times, a Fourier transformation of the spin echo signals gives a projection of spin density on a line parallel to the first magnetic field gradient so as to change phases of respective spins in the direction thereof. The above procedure is repeated for N.sub.z values of the second magnetic field gradient, and Fourier-transformed outputs are obtained, resulting in a matrix of density values of size N.sub.x .times.N.sub.z. Therefore, a formation of a two-dimensional image of a certain plane in the sample can be realized.
In the conventional display method using NMR such as the spin warp method, the time period necessary to form a two-dimensional image of a single plane of a substance is typically on the order of N.sub.z seconds, which is not short enough to form a good image of a human organ such as beating heart.